FA5502 데이터 시트보기 (PDF) - Fuji Electric

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FA5502

Power Supply Control IC

Fuji Electric 

FA5502P/M

(6) ON/OFF control circuit

Fig.12 shows the configuration of the ON/OFF

control circuit. The ON/OFF control circuit consists of a

comparator with hysteresis. To turn the IC from OFF

mode to operating mode, pull up the ON/OFF pin

voltage to 3.95V (typ.) or higher. On the other hand, to

turn the IC from operating mode to OFF mode, pull

down the ON/OFF pin to 2.80V (typ.) or lower.

In the OFF mode, the reference (REF) voltage is cut

off, and the CS pin and OUT pin go approximately 0V.

IC consumption current during OFF mode is 200µA

(max.) which is much smaller than at an operating

mode.

The input current at ON/OFF pin is a very small

value of 500nA.

In the case that external signal is applied to ON/OFF

pin, the ON/OFF pin voltage must not exceed the VCC

pin voltage, even when start up or stop operation.

If ON/OFF operation is not made by external signal,

the ON/OFF pin is normally pulled up to Vcc pin

through 10kΩ to 1MΩ. Then ON/OFF pin voltage goes

to approximately Vcc voltage.

ON/OFF

12

REF circuit

Output circuit

3.95/2.80V

Fig.12 ON/OFF control circuit

(7) Output circuit

As shown in Fig.13, VC pin (pin 9) is configured as

the high power terminal, independent of the IC power

terminal (VCC pin). This pin allows an independent

drive resistance when the power MOSFET is ON and

OFF. Suppose the drive resistance when ON and OFF

are Rg (on) and Rg (off),

Rg(on)=Rg1+Rg2 • • • • • (9)

Rg(off)=Rg2 • • • • • (10)

At standby, the OUT pin is kept Low.

If the drain voltage of power MOSFET oscillates, a

parasitic capacitance between gate and drain may

swing the OUT pin (pin 8) of IC below 0V. If OUT pin

voltage falls below -0.3V, a current may flow to the

parasitic element in IC, whereby the IC may

malfunction. In such a case, Schottky diode must be

connected between OUT pin and GND so as not to

allow a parasitic current to flow to IC.

If VC pin is fed with a source which is independent

of VCC pin, the voltage of VC pin must not exceed that

of VCC pin even start up or stop operation.

Quality is our message

VCC

10

VC

9

Rg1 C5

OUT

8

Rg2

GND

7

Shottky

diode

Fig.13 Output circuit

(8) Undervoltage lockout circuit

This IC contains an undervoltage lockout circuit to

prevent malfunction when the Vcc voltage drops.

When the Vcc voltage rises from 0V, this IC starts

operation at 16.5V (typ.). If the Vcc voltage drops after

the IC starts up, this IC stops operation at 8.9V(typ.).

When IC stops operation by undervoltage lockout

circuit, OUT pin and CS pin is kept low

(9) Compensation circuit for light load

If the output of multiplier and the input of current

error amplifier do not have offset voltage, the input

current to the converter is approximately zero under

condition that the PFC converter operates in no load.

But an actual multiplier and current error amplifier may

have offset voltage. If the offset voltage is negative,

the input current, which corresponds to the offset

voltage, flows into the converter even when the PFC

converter operates in no load. In this case, the PFC

output voltage rises abnormally because of too much

input current.

To avoid these, this IC has an automatic offset

correction circuit for light load. The output voltage of

error amplifier is approximately 1.55V or higher in

normal operation.

If the output voltage drops below 1.55V, this circuit

operates. If there is a negative offset voltage, the

output voltage of error amplifier falls below 1.55V in

the case that the PFC converter operates in no load or

light load. Then, the offset voltage is corrected in the

multiplier circuit. Because of this operation, even

under no load or light load, the PFC output voltage

does not rise abnormally, but is always kept stable.

The amount of correction changes linearly according

to the output of error amplifier, which can make

operation stable.

Fig.14 shows the outline of the effect of this circuit.

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